The present invention relates to a method for depositing an insulating layer in a low pressure chemical vapor deposition (hereinafter referred to as LPCVD) chamber having a high pressure valve. More particularly the present invention relates to a method for performing an NH.sub.3 treatment under operational conditions of high pressure and low temperature as a pre-treatment prior to a process of depositing a silicon nitride layer, to thereby prevent a loss in total operation time.
In general, a silicon nitride layer has been widely used as a gate dielectric, a diffusion mask, or a passivation film for highly integrated circuits due to its high dielectric strength, superior barrier properties to impurity diffusion, and favorable chemical stability.
An NH.sub.3 treatment has also often been carried out as a pretreatment step prior to a process of depositing a silicon nitride layer Si.sub.3 N.sub.4 from a natural oxide layer SiO.sub.2 grown onto a wafer. The NH.sub.3 treatment is usually performed under operational conditions of high temperature (e.g., around 780.degree. C.) and low pressure (e.g., around 0.03 Torr). If the pre-treatment is performed at a temperature lower than 780.degree. C., then the nitridation of the lower layer (e.g., a natural oxide layer) is not effectively accomplished, which can result in inadequate deposition of a silicon nitride layer.
FIGS. 1a and 1b are graphs illustrating changes in the operational conditions during the process of depositing a silicon nitride layer when a conventional NH.sub.3 treatment is performed. In particular, FIG. 1a is a graph illustrating changes of temperature based on elapsed time in a process chamber; and FIG. 1b is a graph illustrating changes of pressure based on elapsed time in the chamber. With reference to the accompanying drawings, a conventional method for depositing a silicon nitride layer has been divided into four steps as follows.
At step one (I), a wafer is loaded into a boat, which is in turn placed into an LPCVD chamber at a stand-by temperature (e.g., 550.degree. C.). At this time, the internal pressure of the chamber is set to remain at a specific pressure (e.g., 760 Torr). The chamber used in this process is preferably designed to allow the internal pressure to be controlled a range of at least 760 Torr to less than about 2.25 Torr (most preferably down to about 0.0 Torr).
At step two (II), in order to bring the internal part of the chamber to a high vacuum state, the chamber is pumped to a low pressure and set at a temperature of up to 780.degree. C. The internal pressure of the chamber is preferably lowered to about 0.0 Torr with its temperature being raised up as high as 780.degree. C. This has the effect of releasing any gas (e.g., vapor) that remains in the chamber. Afterwards, the internal pressure of the chamber is increased up to about 0.3 Torr, a pressure necessary for the NH.sub.3 treatment, and the NH.sub.3 treatment is carried out.
At step three (III), the internal temperature of the chamber is then lowered to a temperature as low as 670.degree. C., and the internal pressure of the chamber is set as high as 0.18 Torr, a pressure necessary for the formation of a silicon nitride layer. Then, under these operational conditions (low temperature of 670.degree. C. and low pressure of 0.18 Torr), a silicon nitride layer is deposited.
At step four (IV), after complete deposition of the silicon nitride layer, the internal pressure of the chamber is again lowered to about 0.0 Torr to release any gas remaining in the chamber. Then, the internal temperature of the chamber is lowered to 550.degree. C., i.e., the initial temperature, and the internal pressure of the chamber is raised back up to 760 Torr, i.e., the initial pressure. At this time, all the processes required for depositing a silicon nitride layer have been completed.
However, if a silicon nitride layer is formed using the aforementioned operational conditions, the silicon nitride deposition method may experience the following problems.
It has been known that the NH.sub.3 treatment is carried out as a pretreatment for the nitridation of a lower layer (e.g., a natural oxide layer formed on a wafer). However, if the pretreatment is performed at the same temperature as the process of forming a silicon nitride layer (670.degree. C. in this example), a nitriding of the lower layer can not be effectively accomplished to a desired extent. Under such parameters, an inadequate deposition level of a silicon nitride layer ill occur. Thus, it is currently necessary to perform the NH.sub.3 treatment at a high temperature of approximately 780.degree. C. to enhance the effectiveness of the nitridation.
However, If the NH.sub.3 treatment is carried out at a high temperature of 780.degree. C., the gas molecules remaining in the chamber can be activated in the course of raising the internal temperature of the chamber from its stand-by temperature of 550.degree. C. up to 780.degree. C. As a result, this increases the pumping time taken to lower the internal pressure of the chamber to its basic vacuum state of about 0.0 Torr before the NH.sub.3 treatment.
In addition, since the internal temperature of the chamber (e.g., 780.degree. C.) is higher for the NH.sub.3 treatment than for the process of depositing a silicon nitride layer (e.g., 670.degree. C.), a sufficiently long period of time is required to decrease the internal temperature of the chamber from 780.degree. C. to 670.degree. C. between these processes. This undesirably increases the operational time required in total for all the processes for depositing the silicon nitride layer.
Since, such a loss in the total operation time may result in reduction in productivity, there is an urgent demand for solving these problems.